1. Field of the Invention
The present invention relates to a resonant switching power supply incorporating an overcurrent protection circuit, which is extensively used as a power supply for electronic equipment like an electronic computer, or as a horizontal deflection circuit of a television receiver.
2. Description of the Prior Art
A switching power supply generally includes, as a switching device, an insulated gate semiconductor device such as a power MOSFET, IGBT or the like, and regulates the direct current output voltage at a fixed voltage by controlling the on/off duration ratio of the switching device. Recently, power supplies of this type have been attracting attention as general purpose power supplies for electronic equipment because components of these power supplies can be miniaturized by turning on and off the switching device at a high frequency. In particular, a resonant switching power supply has been attracting great attention. This is because the resonant switching power supply has distinct characteristics such as low loss and low noise, and can supply a desired voltage both by stepping up or down an input voltage by selecting the turns ratio of a transformer of the power supply. These characteristics arises from the fact that the resonant power supply utilizes the resonance of an isolating transformer whose primary winding is connected in parallel with a resonant capacitor and hence the switching voltage and current take the shape of a sinusoidal waveform, thereby preventing sudden changes in the voltage and current.
FIG. 1 is a circuit diagram showing a conventional switching power supply. In this figure, a switching transistor 1 is connected with a diode 1A in inverse-parallel fashion, and the collector C of the transistor 1 is connected to a second terminal of a primary winding 2A of a transformer 2. The primary winding 2A is connected in parallel with a resonant capacitor 3. A first terminal of the primary winding 2A and the emitter E of the transistor 1 are connected to the positive and negative terminals of a direct current power supply 4, respectively. The direct current power supply 4, includes a bridge rectifier 4A and a smoothing capacitor 4C. The above-mentioned components constitute a primary main circuit.
On the other hand, a secondary winding 2B of the transformer 2 is connected to a direct current output circuit 5 including a full-wave rectifier 5A, a reactor 5B and a capacitor 5C so as to supply a direct current output to an external load.
A control circuit 6 supplies the base B of the switching transistor 1 with a driving signal 6S in the form of a rectangular pulse train through a serial resistor 14, and controls the on/off duration ratio of the switching transistor 1, thereby regulating the output voltage V2 of the switching power supply at a fixed voltage or a rated voltage.
More specifically, when the primary current of the transformer 2 is turned on and off by the switching transistor 1, a resonance voltage V1 with a sinusoidal waveform is generated in the primary winding 2A. Here, the waveform of the resonance voltage V1 is determined by the product of the leakage inductance of the primary winding 2A and the capacitance of the resonance capacitance 3. This resonance voltage induces in the secondary winding 2B an alternating positive and negative current which is rectified by the pair of diodes 5A, thus producing the direct current output voltage V2.
When the switching transistor 1 is turned off after it has been turned on so that a current flows through the transformer 2, a positive resonance voltage is applied to the collector of the transistor 1, and subsequently, a negative resonance voltage is applied to the collector. When the negative resonance voltage is applied to the switching transistor 1, the parallel diode 1A turns on, which turns off the switching transistor 1 during this period because the voltage applied across the collector C and emitter E is removed. Thus, the collector-to-emitter voltage (called collector voltage hereinafter) assumes a waveform similar to a sinusoidal waveform, preventing a sudden change of the voltage. In addition, since the switching transistor 1 turns off near zero cross points of the voltage, a power supply with low switching loss and noises is achieved.
In the switching power supply, when an overcurrent exceeding the rated current flows through the secondary of the transformer 2 owing to a discharge or the like across terminals of an external load, the corresponding primary current Ic abruptly increases, which in turn increases the primary voltage V1 in direct proportion to the current. Thus, an overcurrent and overvoltage exceeding the rated values are generated across the collector and the emitter of the switching transistor 1.
To protect the switching transistor 1 from damage by restricting such an overcurrent and overvoltage, a surge absorber 50 is connected in parallel with the resonant capacitor 3. In addition, at the secondary of the transformer 2, an overcurrent protective circuit 12 is provided. The overcurrent protective circuit 12, receiving a detected voltage from a current detection resistor 11 serially connected to the output circuit 5 to detect the overcurrent, performs inverse amplification of the detected voltage exceeding a normal value, and applies the output to a second input terminal of the AND gate 13A as a gate signal 12S. Thus, the drive signal 6S applied to a first input terminal of the AND gate 13A is stopped by the AND gate 13A, thereby restricting the overcurrent. The output terminal of the AND gate 13A is connected to the drive resistor 14 via an isolating transformer 13B so that the adverse effect of the voltage difference between the primary and secondary of the transformer 2 is eliminated.
The overcurrent protective circuit 12 temporarily interrupts the output of the switching power supply when the overcurrent is detected. Hence, in a switching power supply for electronic equipment which is adversely affected by an instantaneous break, the response speed of the overcurrent protective circuit is lowered, and the protective level against an overcurrent is set relatively high above the rated current. As a result, the surge absorber 50 is needed to protect the switching transistor 1 from damage. In addition, since the overcurrent is detected at the secondary of the transformer 2, the isolating transformer 13B is needed to isolate the overcurrent protective circuit 12 from the primary circuit. Accordingly, as the number of components increases, a more complicated circuit arrangement is required.
Furthermore, since the protective level against the overcurrent is made high, a switching device having a larger current capacity and higher withstanding voltage than is determined by the rated values must be used as the switching transistor 1. This poses another problem in that the switching power supply becomes expensive. Moreover, using a device of a high withstanding voltage hinders achieving a miniaturized, high performance power supply by adopting a switching device of a low saturation voltage and high switching speed, which presents still another problem.